Skip to content

RemoveUnusedBrs: When turning an if-br into br_if, consider the value's cost #9187

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
kripken merged 3 commits into from
Oct 1, 2026
Merged

RemoveUnusedBrs: When turning an if-br into br_if, consider the value's cost #9187

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
9e60f69
start
kripken Oct 1, 2026
6bc808d
fix
kripken Oct 1, 2026
007a6d6
finish
kripken Oct 1, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
17 changes: 12 additions & 5 deletions src/passes/RemoveUnusedBrs.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -65,9 +65,15 @@ stealSlice(Builder& builder, Block* input, Index from, Index to) {
return ret;
}

// to turn an if into a br-if, we must be able to reorder the
// condition and possible value, and the possible value must
// not have side effects (as they would run unconditionally)
// Check if a single expression is too costly to move from a conditional to an
// unconditional place.
static bool tooCostlyToRunUnconditionally(const PassOptions& passOptions,
Expression* curr);

// To turn an if into a br-if, we must be able to reorder the condition and
// possible value, and the possible value must not have side effects (as they
// would run unconditionally). Also, if there is a value, it must not be too
// expensive to run unconditionally.
static bool canTurnIfIntoBrIf(Expression* ifCondition,
Expression* brValue,
PassOptions& options,
Expand All @@ -79,6 +85,9 @@ static bool canTurnIfIntoBrIf(Expression* ifCondition,
if (!brValue) {
return true;
}
if (tooCostlyToRunUnconditionally(options, brValue)) {
return false;
}
EffectAnalyzer value(options, wasm, brValue);
if (value.hasSideEffects()) {
return false;
Expand Down Expand Up @@ -120,8 +129,6 @@ static bool tooCostlyToRunUnconditionally(const PassOptions& passOptions,
}
}

// As above, but a single expression that we are considering moving to a place
// where it executes unconditionally.
static bool tooCostlyToRunUnconditionally(const PassOptions& passOptions,
Expression* curr) {
// If we care entirely about code size, just do it for that reason (early
Expand Down
89 changes: 88 additions & 1 deletion test/lit/passes/remove-unused-brs-gc.wast
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -997,4 +997,91 @@
(i32.const 0)
)
)
)

;; CHECK: (func $costly-br-value-no-if (type $16) (param $x i32) (result anyref)
;; CHECK-NEXT: (block $out (result anyref)
;; CHECK-NEXT: (if
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: (then
;; CHECK-NEXT: (br $out
;; CHECK-NEXT: (struct.new_default $struct)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (if
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: (then
;; CHECK-NEXT: (br $out
;; CHECK-NEXT: (struct.new_default $struct)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (ref.null none)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $costly-br-value-no-if (param $x i32) (result anyref)
;; We do not turn an if into a br_if if that makes a costly value execute
;; unconditionally.
(block $out (result anyref)
(if
(local.get $x)
(then
(br $out
;; An allocation is too expensive to unconditionalize.
(struct.new_default $struct)
)
)
)
;; Another if, so the entire block is not trivially optimized in another way.
(if
(local.get $x)
(then
(br $out
(struct.new_default $struct)
)
)
)
(ref.null any)
)
)

;; CHECK: (func $cheap-br-value-yes-if (type $17) (param $x i32) (result i32)
;; CHECK-NEXT: (block $out (result i32)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (br_if $out
;; CHECK-NEXT: (i32.const 10)
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (br_if $out
;; CHECK-NEXT: (i32.const 20)
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (i32.const 42)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
(func $cheap-br-value-yes-if (param $x i32) (result i32)
;; For comparison to above, if the br value is cheap, we do emit br_ifs here.
(block $out (result i32)
(if
(local.get $x)
(then
(br $out
(i32.const 10)
)
)
)
;; Another if, so the entire block is not trivially optimized in another way.
(if
(local.get $x)
(then
(br $out
(i32.const 20)
)
)
)
(i32.const 42)
)
))
Loading